Role of Water and Oxygen Molecules in the Lubricity of Carbon Nitride Coatings under a Nitrogen Atmosphere

Abstract

In this study, an experimental apparatus was developed where the relative humidity (RH) and oxygen concentration could be controlled between 0.01-10%RH and 1-100000 ppm, respectively. This apparatus was used to study the roles of water and oxygen molecules on the friction of carbon nitride (CN X) coatings. The friction properties of Si 3 N 4 balls sliding against CN X (Si 3 N 4 /CN X) under a nitrogen atmosphere were classified into the following three modes according to their average friction coefficients (µ a) and the stability of the friction: Mode I, µ a < 0.05 (stable); Mode II, 0.05 < µ a < 0.3 (stable); and Mode III, 0.3 < µ a (unstable). To achieve a low friction coefficient (e. g. < 0.05), the optimum RH and oxygen concentration were 0.1-1.0%RH and 100-10000 ppm, respectively. Time-of-flight secondary ion mass spectroscopy (TOF-SIMS) showed that hydrogen and hydroxyl radicals derived from water molecules chemisorbed onto the worn surfaces of the CN X coatings. Raman spectroscopy suggested that the structure of carbon on the worn surfaces was different from that of the deposited CN X coating. We concluded that low friction (Mode I) arose from the water and oxygen molecules terminating carbon dangling bonds and structural changes in CN X .

Impact of Relative Humidity on the Formation of Low-Frictional Interface and its Continuity in Tribological Systems with Hydrogenated Carbon Nitride Coatings

“…The spectra showed a growth in the D peak and a shift in the G peak toward a higher wave shift compared with that of the initial CN x :H. This change in the Raman spectrum indicates different amorphous structure of the carbonaceous tribolayer from the initial coatings, indicating a transformation into a more graphitic structure. This spectral pro le is similar to that of low-frictional tribolayers with nanometer-scale thickness (less than 10 nm) previously reported in our studies, suggesting a comparable carbon structure [19,[22][23][24]. Similar to those earlier ndings, the carbonaceous tribolayer detected here is also exceedingly thin for optical microscopy detection, supporting its identity as the critical low-frictional tribolayer essential for reducing friction in CN x :H systems, with an inferred thickness of less than 10 nm [19,24].…”

“…Upon setting the test specimens, the air atmosphere (20-21% O 2 ) inside the case was consistently maintained within the 5-50% RH range using the dehumidi er. For friction tests in air or nitrogen atmospheres with relative humidity ranging from 0.01 to 5% RH, the glovebox-type apparatus was used, as detailed in our previous research [22]. The friction test system was installed within a glovebox (UN-650L, UNICO, Japan), which allowed for control over the relative humidity and oxygen concentration within the chamber using a gas puri er.…”

“…Carbon nitride (CN x , a-C:N) and hydrogenated carbon nitride (CN x :H, a-C:N:H) represent promising types of hard carbon coatings known for their excellent tribological properties in dry, lubrication-free environments [17,18]. Numerous studies have reported that CN x and CN x :H coatings consistently achieve a stable friction coe cient of less than 0.05 in dry environments such as nitrogen atmosphere [19][20][21][22][23][24][25][26][27][28]. A critical factor in achieving low friction within these tribological systems involves the formation of a low-frictional carbonaceous tribolayer on the mating surface.…”

“…Moreover, environmental water molecules can impact the state of hydrogen on the friction surface and alter the friction properties of hard carbon coatings. Yamada reported an optimal relative humidity for achieving low friction (µ < 0.05) between CN x and Si 3 N 4 , where environmental water molecules are selectively chemisorbed on the friction surface as diverse hydrogen and hydroxyl groups at the interface displaying low friction [22]. Similarly, numerous studies indicate that the friction properties of hard carbon coatings are in uenced by relative humidity under lubrication-free conditions.…”

Superlubricity with Graphitization in Ti-Doped DLC/Steel Tribopair: Response on Humidity and Temperature

“…Kato and Adachi and Yamada et al revealed that O 2 is a harmless chemical in N 2 for changing friction for self-mated carbon nitride which is one coating type among HF-DLC; however, water vapor affected increasing CoF and wear. Therefore, ta-C/ta-C and Ti-DLC/steel tribopairs could provide low friction in dry environments owing to the establishment of a C / C contact interface.…”